System for preventing the formation of martensite in metals joining applications

ABSTRACT

A system for joining materials such as metal alloys that includes a first component, wherein the first component includes a first alloy having a known austenization temperature below which martensite forms when the component is heated and then cooled at a predetermined rate of cooling; a second component, wherein the second component includes a second alloy; and a welding apparatus operative to create a weld between the first and second components without crossing the austenization temperature of the first alloy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/697,675 filed on Sep. 6, 2012, and entitled“System and Method for Joining Materials,” the disclosure of which isincorporated by reference for all purposes, as if fully rewrittenherein.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention was supported in part by contract numberDTFR53-12-C-00002 from the Federal Railroad Administration, TrackResearch Division. The U.S. Government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

The present invention relates generally to a system and method forjoining two components to one another, particularly where one of the twocomponents is a hardened metal, and more specifically to a system andmethod for attaching a stud or other appurtenance to a specific locationon a length of steel rail for the purpose of attaching a signal wire tothe stud.

Railroad signaling systems are essential for enabling safe and efficientmovement of rail traffic. Many modern railroad signal systems employ atrack circuit to detect the presence of a train within a section oftrack known as a signal block. The basic principle behind the trackcircuit involves the connection of the two rails by the wheels and axleof locomotives to short out an electrical circuit. This circuit ismonitored by electrical equipment to detect the absence of the trains.An integral part of the track circuit is the two parallel running railson which a train runs. Various types of signal devices are typicallyconnected to these rails to complete the track circuit. Known techniquesfor connecting a wire to a rail include exothermic welding processeswhere the wire is welded to the rail. Other techniques includecompressing a metal sleeve including the wire in a hole drilled in therail or clamping a wire directly to the rail. Many signaling systemincidents are known to be caused by failures at the rail-wire interface,where track wires, bond wires, or propulsion-current bond wires areattached to the rails to provide an electrical path for controllingtrain control signals. These failures contribute to train delays andadditional maintenance costs for emergency and unplanned repairs and arehighly undesirable for these and other reasons.

As indicated above, signal wires are attached to rails to allow forpositive train control and to sense breaks in the rails for avoidingaccidents. A reliable signal wire-to-rail connection is essential forsignal system functionality and failures cause service disruptions andcan affect the integrity of the rail, leading to rail failure. Currentlyused methods for attaching a signal wire to a length of track involvethe use of an appurtenance or stud that is attached directly to therail. The signal wire is then attached or connected to the stud. Commonattachment methodologies include brazing, soldering, drilling, and/orclamping the stud/wire to the rail. Many brazing methods requirepreheating the rail section to which the stud will be attached and thenprecisely controlling the rate of cooling to avoid the undesirableformation of untempered martensite in the rail. With brazingmethodologies there is also the risk of liquid metal embrittlement asthe rail is under tensile stress to maintain neutral temperature and aliquid metal is present during the process. Accordingly, it is a commonprecautionary practice to locate the studs at the neutral axis of therail due to the possible formation of a brittle layer around the jointcaused by overheating of the stud/wire to rail connection point. Theplacement of welds/braze joints on the head of the rail is known to haveresulted in the formation of martensite in the head of the rail, whichinitiated cracks that led to several train derailments; thus the neutralaxis is generally safer from a catastrophic failure perspective.However, placement at this location makes the wire harnesses susceptibleto snagging by maintenance equipment and the formation of martensite inthis area may still lead to cracking and rail failure. Furthermore, mostknown attachment methodologies require a degree of operator skill, theabsence of which may result in inconsistent or incorrect installationsand ultimately in failure of the stud/wire connection, particularly inmass production. Thus, there is an ongoing need for an improved systemand method for attaching a stud or appurtenance to a specific locationon a length of steel rail.

SUMMARY OF THE INVENTION

The following provides a summary of certain exemplary embodiments of thepresent invention. This summary is not an extensive overview and is notintended to identify key or critical aspects or elements of the presentinvention or to delineate its scope.

In accordance with one aspect of the present invention, a first systemfor joining materials is provided. This system includes a firstcomponent, wherein the first component includes a first alloy having aknown austenization temperature below which martensite forms when thecomponent is heated and then cooled at a predetermined rate of cooling;a second component, wherein the second component includes a secondalloy; and a welding apparatus operative to create a weld between thefirst and second components without crossing the austenizationtemperature of the first alloy.

In accordance with another aspect of the present invention, a secondsystem for joining materials is provided. This system includes a firstcomponent, wherein the first component includes a first alloy having aknown austenization temperature below which martensite forms when thecomponent is heated and then cooled at a predetermined rate of cooling;a second component, wherein the second component includes at least onelow-sulfur, low-lead alloy; and a welding apparatus operative to createa weld between the first and second components without crossing theaustenization temperature of the first alloy.

In yet another aspect of this invention, a third system for joiningmaterials is provided. This system includes a first component, whereinthe first component includes a first alloy having a known austenizationtemperature below which martensite forms when the component is heatedand then cooled at a predetermined rate of cooling, wherein the firstcomponent further includes heat-treated steel, and wherein the firstcomponent is a length of train rail; a second component, wherein thesecond component includes at least one low-sulfur, low-lead alloy,wherein the second component further includes an appurtenance that isoperative to provide a surface to which a signal wire or other devicemay be attached, and wherein the appurtenance is a stud, bolt, or block;and a welding apparatus operative to create a weld between the first andsecond components without crossing the austenization temperature of thefirst alloy, wherein the welding apparatus is operative to performsolid-state welding, and wherein the welding apparatus is an inertiafriction welding machine.

Additional features and aspects of the present invention will becomeapparent to those of ordinary skill in the art upon reading andunderstanding the following detailed description of the exemplaryembodiments. As will be appreciated by the skilled artisan, furtherembodiments of the invention are possible without departing from thescope and spirit of the invention. Accordingly, the drawings andassociated descriptions are to be regarded as illustrative and notrestrictive in nature.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying images, which are incorporated into and form a part ofthe specification, illustrate one or more exemplary embodiments of theinvention and serve to explain the principles of the invention, andwherein:

FIG. 1 is a side view of a portable inertia friction welder mounted on alength of rail in accordance with an exemplary embodiment of thisinvention;

FIGS. 2 a-b are front and side views respectively, of an exemplary studor appurtenance suitable for mounting on the length of rail shown inFIG. 1;

FIG. 3 provides a high-magnification metallurgical section of a weldjoint created by the method of the present invention showing no apparentchange to the underlying rail steel material; and

FIG. 4 provides a macro view of a weld stud mounted on rail material inaccordance with the method of the present invention.

DESCRIPTION OF THE INVENTION

Certain exemplary embodiments of the present invention are now describedwith reference to the Figures. Although the following detaileddescription contains many specifics for purposes of illustration, aperson of ordinary skill in the art will appreciate that many variationsand alterations to the following details are within the scope of theinvention. Accordingly, the following embodiments of the invention areset forth without any loss of generality to, and without imposinglimitations upon, the claimed invention.

As previously stated, the present invention relates generally to asystem and method for joining two components to one another,particularly where one of the two components is a hardened metal, andmore specifically to a system and method for attaching a stud orappurtenance to a specific location on a length of steel rail for thepurpose of attaching a signal wire to the stud. The system and method ofthis invention have applicability beyond use with rails and signal wiresbecause the process can also be used to attach appurtenances tohardenable materials without negatively affecting the underlyingmaterial properties. For example, attaching a bolt on location to ahardened cutting surface that is heat treated would be possible withoutdeleterious effects to the pre-hardened material. Essentially, thesystem and method of this invention may be used for any number ofapplications that require the attachment of one metal component toanother metal component wherein the use of more traditional joining orwelding techniques would potentially damage the substrate metal. Usingthe system and method of this invention, the risk of liquid metalembrittlement and martensite formation are substantially eliminatedbecause there is no melting of a stud or braze material to infiltratedthe grain boundaries in the steel and the welding temperature is keptbelow the critical transformation temperature of hardened steel. Variousexemplary embodiments of this invention are described in greater detailbelow.

The present invention typically includes two or more metal componentsthat are joined together using a welding technique that avoids damagingeither component during the joining process. In one exemplaryembodiment, the first component is a length of hardened steel rail usedfor train track. Modern track typically uses hot rolled steel with aprofile of an asymmetrical rounded I-beam. Unlike some other uses ofiron and steel, railway rails are subject to very high stresses and aretypically made of very high-quality steel alloy. Attachment of anappurtenance to heat-treated steel is typically very difficult due thenature of this alloy. The second component is a lower-strengthappurtenance such as a stud to which signal wire is or will be attached.This stud is joined to the steel rail at a desired location using awelding process, such as friction welding, which prevents the occurrenceof liquid metal embrittlement in the rail alloy. Friction welding is asolid-state welding process that generates heat through mechanicalfriction between a moving workpiece and a stationary component, with theaddition of a lateral force called “upset” to plastically displace andfuse the materials. The combination of fast joining times (on the orderof a few seconds), and direct heat input at the weld interface, yieldsrelatively small heat-affected zones. Friction welding techniques aregenerally melt-free, which avoids grain growth in engineered materials,such as high-strength, heat-treated steels. Another advantage offriction welding is that it allows dissimilar materials to be joined toone another. Normally, the wide difference in melting points of twodissimilar materials would make it nearly impossible to weld usingtraditional techniques, and would require some sort of mechanicalconnection. Friction welding provides a “full strength” bond with noadditional weight.

With regard to a steel rail component, the system and method of thepresent invention produces a weld on rail steel without crossing theknown austenitization temperature for such steel, thereby avoiding theneed for tempering post weld or controlled cooling to avoid martensiteformation. Austenitization involves heating iron, an iron-based metal,or steel to a temperature at which it changes crystal structure fromferrite to austenite. Martensite refers to a very hard form of steelcrystalline structure and is formed by rapid cooling (quenching) ofaustenite which traps carbon atoms that do not have time to diffuse outof the crystal structure. This martensitic reaction begins duringcooling when the austenite reaches a known martensite start temperatureand the parent austenite becomes mechanically unstable. Since quenchingcan be difficult to control, many steels are quenched to produce anoverabundance of martensite and then tempered to gradually reduce itsconcentration until the right structure for the intended application isachieved. Too much martensite leaves steel brittle, too little leaves itsoft. With regard to an appurtenance or stud component, certainmaterials (e.g., low-sulfur, low-lead alloys) exhibit highly desirablecharacteristics, which permit friction welding of the stud to the railwithout a temperature in excess of the austenitization temperature ofthe rail or steel without demanding thrust loads that are too high for aportable inertia welding system. The use of a hexagonal shaped stud witha circular face minimizes machining cost for the stud and provideswrench flats during in-service work.

One embodiment of this invention utilizes a portable battery-poweredinertia friction welder that is mounted on a length of rail forlow-energy input welding. The use of a low mass flywheel coupled withhigh surface velocity allows for a lightweight portable unit. Thecontrols for speed and thrust load control are specific to the machinetool mounted to the rail. The use of a preloaded spring assembly or aprecharged air or hydraulic cylinder provides weld force/thrust load inthe portable system and a pin/ball release holds the thrust load on thespring. A lightweight clamp attaches the system to the head of the railand a locator placed under the ball of the rail ensures repeatableplacement on the rail regardless of wear condition. Location for thestud is dictated by reference to the under head radius region thattransitions into the web of the rail. This configuration allows forreliable installation of studs with little operator influence on theprocess as all critical parameters are predetermined and mechanicallycontrolled. As shown in FIG. 1, an exemplary embodiment of weldingsystem 10 includes a length of rail 12 upon which restraining clamp 10is mounted. Chuck 16 houses the stud (see FIGS. 2 a-b) or otherappurtenance, and flywheel 18 is connected to chuck 16. Front mountguide plate 20 supports bearing end plate 22, which is connected tobearing assemblies 24 which provide motor decouple and force engagementcontrol. Rear mount guide plate supports bearing assemblies 24 andspring pack 28, which includes an air cylinder for providing thrustload. Motor coupler 32 is connected to drive shaft 34, which passesthrough back up plate 30 and connects with spring pack 28.

In accordance with this invention, welds may be produced using anportable inertia friction welding machine such as that disclosed in U.S.Pat. No. 6,779,709 (Stotler et al.), which is incorporated by referenceherein, in its entirety, for all purposes. The device disclosed in U.S.Pat. No. 6,779,709 is referred to as the m120 inertia friction welderand is a stationary programmable inertia friction welding machine thatis capable of varying thrust load from 2000 lbs. to 24,000 lbs.; varyingrotating mass from 1.21 wk² to 19 wK², and varying initial spindle speedfrom 300 to 13,000 RPM. This device typically uses collet type clamps tohold parts and or tooling in the headstock and tailstock. Additionally,surface velocity, thrust load, and inertia may be varied to control heatinput. Suitable alloys for the appurtenance (i.e., stud) includelow-sulfur, low-lead alloys generally and C464 Naval Brass, C172 Class 4copper, C260 Brass, Muntz Metal (National Bronze and Metals, Houston,Tex.; Southern Copper, Pelham, Ala.) and Ni-12P braze alloy,specifically. Studs such as that shown in FIGS. 2 a-b, can be made from½-in hex stock to allow for easy torsion energy delivery and to simplifyproduction. Weld strengths approached 6000 pounds for the hex partswelded which is roughly 35ksi tensile strength or 50% of the cold workedC464 brass. With a hex stud design, a speed of 4000-4500 rpms, aninertial mass of 1.21 WK², and a thrust load of 5200-lbs force to6000-lbs force may be employed with work hardened Naval Brass. FIGS. 1-2show metallographic and SEM analysis conducted on weld joints to verifythat no martensite was formed due to the fact that the criticaltemperature in the steel was not exceeded during welding. Pretinnedstuds or appurtenances with an alloy to create a solid state braze jointsuch as 50/50 Pb—Sn solder, are also compatible with this invention. Aswill be appreciated by one of ordinary skill in the art, the process offriction welding is scalable based on variables such as the surface areaand mean diameter of the appurtenance (i.e., second component).

Advantageously, the present invention permits the installation of signalwires and studs on the head of the rail as no deleterious effects occurto the underlying rail material, i.e., brittleness and other weaknessesdo no occur (see FIGS. 3-4). Currently, signal wires are located on theneutral axis of the rail making these connections susceptible to damageby maintenance equipment that snags and breaks studs and wiring locatedon the web neutral axis of the rail. Thus, the point of attachment maybe moved from a low stress area to a critically loaded area (i.e., highstress area) without creating noticeable changes in the microstructureof the underlying alloy or problems with the integrity and strength ofunderlying substrate metal.

While the present invention has been illustrated by the description ofexemplary embodiments thereof, and while the embodiments have beendescribed in certain detail, it is not the intention of the Applicant torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to any of the specific details, representativedevices and methods, and/or illustrative examples shown and described.Accordingly, departures may be made from such details without departingfrom the spirit or scope of the applicant's general inventive concept.

What is claimed: 1) A system for joining materials, comprising: (a) a first component, wherein the first component includes a first alloy having a known austenization temperature below which martensite forms when the component is heated and then cooled at a predetermined rate of cooling; (b) a second component, wherein the second component includes a second alloy; and (c) a welding apparatus operative to create a weld between the first and second components without crossing the austenization temperature of the first alloy. 2) The system of claim 1, wherein the first component further includes heat-treated steel. 3) The system of claim 1, wherein the first component is a length of train rail. 4) The system of claim 1, wherein the second component further includes an appurtenance, wherein the appurtenance is operative to provide a surface to which a signal wire or other device may be attached. 5) The system of claim 4, wherein the appurtenance is a stud, bolt, or block. 6) The system claim 1, wherein the second component further includes at least one low-sulfur, low-lead alloy. 7) The system claim 1, wherein the second component further includes C464 Naval Brass, C172 Class 4 copper, C260 Brass, Muntz Metal. Ni-12P braze alloy, or combinations thereof. 8) The system of claim 1, wherein the welding apparatus is operative to perform solid-state welding. 9) The system of claim 1, wherein the welding apparatus is an inertia friction welding machine. 10) The system of claim 9, wherein the second component is a hex stud that further includes C464 brass, and wherein the inertia friction welding machine operates at a speed of 4000-4500 rpms, an inertial mass of 1.21 WK², and a thrust load of about 5200-lbs force to 6000-lbs force. 11) A system for joining materials, comprising: (a) a first component, wherein the first component includes a first alloy having a known austenization temperature below which martensite forms when the component is heated and then cooled at a predetermined rate of cooling; (b) a second component, wherein the second component includes at least one low-sulfur, low-lead alloy; and (c) a welding apparatus operative to create a weld between the first and second components without crossing the austenization temperature of the first alloy. 12) The system of claim 11, wherein the first component further includes heat-treated steel. 13) The system of claim 11, wherein the first component is a length of train rail. 14) The system of claim 11, wherein the second component further includes an appurtenance, wherein the appurtenance is operative to provide a surface to which a signal wire or other device may be attached, and wherein the appurtenance is a stud, bolt, or block. 15) The system claim 11, wherein the second component further includes C464 Naval Brass, C172 Class 4 copper, C260 Brass, Muntz Metal. Ni-12P braze alloy, or combinations thereof. 16) The system of claim 11, wherein the welding apparatus is operative to perform solid-state welding, and wherein the welding apparatus is an inertia friction welding machine. 17) The system of claim 16, wherein the second component is a hex stud that further includes C464 brass, and wherein the inertia friction welding machine operates at a speed of 4000-4500 rpms, an inertial mass of 1.21 WK², and a thrust load of about 5200-lbs force to 6000-lbs force. 18) A system for joining materials, comprising: (a) a first component, wherein the first component includes a first alloy having a known austenization temperature below which martensite forms when the component is heated and then cooled at a predetermined rate of cooling, wherein the first component further includes heat-treated steel, and wherein the first component is a length of train rail; (b) a second component, wherein the second component includes at least one low-sulfur, low-lead alloy, wherein the second component further includes an appurtenance that is operative to provide a surface to which a signal wire or other device may be attached, and wherein the appurtenance is a stud, bolt, or block; and (c) a welding apparatus operative to create a weld between the first and second components without crossing the austenization temperature of the first alloy, wherein the welding apparatus is operative to perform solid-state welding, and wherein the welding apparatus is an inertia friction welding machine. 19) The system claim 18, wherein the second component further includes C464 Naval Brass, C172 Class 4 copper, C260 Brass, Muntz Metal. Ni-12P braze alloy, or combinations thereof. 20) The system of claim 18, wherein the second component is a hex stud that further includes C464 brass, and wherein the inertia friction welding machine operates at a speed of 4000-4500 rpms, an inertial mass of 1.21 WK², and a thrust load of about 5200-lbs force to 6000-lbs force. 